The present invention relates to a biaxially oriented polyester film having excellent wear resistance and scuff resistance. More particularly, the present invention relates to a biaxially oriented polyester film which is suitable for a magnetic recording medium.
Polyester films, which are excellent in physical and chemical properties, have been widely used as a base material of various products. Above all, a biaxially oriented polyethylene terephthalate film, which is excellent especially in mechanical strength, dimensional stability and flatness, is used for various purposes including a base film of a magnetic recording medium and a capacitor dielectric.
The major problem in a film for a magnetic recording medium which has been increasingly and widely used is in wear resistance thereof.
When a film comes into contact with a metal pin at a high speed, the friction therebetween becomes higher to cause scuffs on the film and generation of powder (worn-out powder) scraped off the film by wear.
The present inventor has proposed an improvement in wear resistance of a film in Japanese Patent Application Laid-Open (KOKAI) No. 1-306220 (1989), etc. where particles of aluminum oxide having a high Mohs hardness are mixed in the film.
The present inventor, however, has found that when particles such as aluminum oxide particles are mixed in a film, a new problem is brought about. While a film containing those particles exhibits a very excellent effect on the prevention of the scuffing of the film and the production of worn-out powder during the contact with a metal pin, when the film coated with a magnetic layer thereon is wound up, the particles existing on the surface of the base film which comes into contact with the magnetic layer often cause scuffs on the magnetic layer surface to deteriorate the electromagnetic properties. This disadvantage is more remarkable when the condition for orientating a film is strict, i.e., stretch ratio in the machine direction or transverse direction is larger.
As a result of researches for solving this problem, the present inventor has found that the use of particular agglomerate of aluminum oxide particle can effectively solve the problem. The present invention has been accomplished based on this finding.